Is there a GT cover that I can use to prevent two adjacent electrical cables from connecting? Or any other "zero-thickness" divider I can place in between them?
You're in luck! The very cheap "wood plank" type covers (eg "Spruce Plank (can be used as cover)") can be placed on any face of a machine or cable and prevent electrical connections.
They use the gregtech cover placement rules, so click in the centre of a face to place on that face, or use the edge segments to say which other face to place the cover on.